Process for the fabrication of an inertial sensor with failure threshold

ABSTRACT

A process for the fabrication of an inertial sensor with failure threshold includes the step of forming, on top of a substrate of a semiconductor wafer, a sample element embedded in a sacrificial region, the sample element configured to break under a preselected strain. The process further includes forming, on top of the sacrificial region, a body connected to the sample element and etching the sacrificial region so as to free the body and the sample element. The process may also include forming, on the substrate, additional sample elements connected to the body.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a process for the fabrication of aninertial sensor with failure threshold.

2. Description of the Related Art

As is known, modern techniques of micromachining of semiconductors canbe advantageously exploited for making various extremely sensitive andprecise sensors, having further small overall dimensions. The so-calledMEMS sensors (or micro-electro-mechanical-system sensors), are sensorsthat can be integrated in a semiconductor chip and are suitable fordetecting various quantities. In particular, both linear and rotationalMEMS accelerometers with capacitive unbalancing are known. In brief,these accelerometers are normally provided with a fixed body and of amobile mass, both of which are conductive and are capacitively coupledtogether. In addition, the capacitance present between the fixed bodyand the mobile mass may vary, and its value depends upon the relativeposition of the mobile mass with respect to the fixed body. When theaccelerometer is subjected to a stress, the mobile mass is displacedwith respect to the fixed body and causes a variation in the couplingcapacitance, which is detected by a special sensing circuit.

As mentioned previously, MEMS accelerometers are extremely sensitive andprecise; however, they are not suitable for being used in manyapplications, mainly because they are complex to make and their cost isvery high. On the one hand, in fact, the processes of fabricationinvolve the execution of numerous non-standard steps and/or the use ofnon-standard substrates (for example, SOI substrates); on the otherhand, it is normally necessary to provide feedback sensing circuitsbased upon differential charge amplifiers, the design of whichfrequently involves some difficulties.

In addition, in many cases the precision of capacitive MEMS sensors isnot required and, indeed, it is not even necessary to have aninstantaneous measurement of the value of acceleration. On the contrary,it is frequently just necessary to verify whether a device incorporatingthe accelerometer has undergone accelerations higher than a pre-setthreshold, normally on account of impact. For example, the majority ofelectronic devices commonly used, such as cell phones, are protected bya warranty, which, however, is no longer valid if any malfunctioning isdue not to defects of fabrication but to an impact consequent on thedevice being dropped onto an unyielding surface or in any case on a usethat is not in conformance with the instructions. Unless visible damageis found, such as marks on the casing or breaking of some parts, it ispractically impossible to demonstrate that the device has suffereddamage that invalidates the warranty. On the other hand, portabledevices, such as cell phones, exactly, are particularly exposed to beingdropped and consequently to getting broken, precisely on account of howthey are used.

Events of the above type could be easily detected by an inertial sensor,which is able to record accelerations higher than a pre-set threshold.However, the use of MEMS accelerometers of a capacitive type in thesecases would evidently lead to excessive costs. It would thus bedesirable to have available sensors that can be made using techniques ofmicromachining of semiconductors, consequently having overall dimensionscomparable to those of capacitive MEMS sensors, but simpler as regardsboth the structure of the sensor and the sensing circuit. In addition,also the processes of fabrication should be, as a whole, simple andinexpensive.

BRIEF SUMMARY OF THE INVENTION

The purpose of the present invention is to provide a process for thefabrication of an inertial sensor with failure threshold, which willenable the problems described above to be overcome.

According to an embodiment of the present invention, a process isprovided for the fabrication of an inertial sensor with failurethreshold, including the step of forming at least one sample elementembedded in a sacrificial region on top of a substrate of asemiconductor wafer, the sample element being configured to fractureunder a preselected force. The process further includes forming, on topof the sacrificial region, a body connected to the sample element, andetching the sacrificial region, so as to free the body and the sampleelement.

The process may include the step of making a weakened region of thesample element. The weakened region may be made by forming a narrowedregion or notches in the sample element.

The process may include forming a plurality of sample elements, eachconfigured to fracture under the preselected force.

According to an alternative embodiment of the invention, a method formanufacturing an inertial sensor is provided, comprising forming, on asemiconductor substrate, a sample element configured to break under apreselected strain, the sample element having a first end coupled to thesubstrate, and forming, above the semiconductor substrate, asemiconductor material body coupled to a second end of the sampleelement. The method may include forming a weakened region on the sampleelement, with the sample element configured to break at the weakenedregion under the preselected strain.

According to this embodiment, the sample element may have a T shape, thefirst end being the cross-bar portion of the T and being coupled to thesubstrate at extreme ends thereof, the second end being the uprightportion of the T.

The method may also include forming an additional sample element havinga first end coupled to the substrate, a second end coupled to thesemiconductor material body, and configured to break under thepreselected strain.

According to another embodiment of the invention, A method of measuringmovement of a device is provided, including providing a circuit in thedevice configured to permanently change the conductive state of aconductive path in the event the device is subjected to an accelerationexceeding a preselected level, applying a potential at first and secondends of the conductive path, and detecting a change in the conductivestate of the conductive path.

The method may further include breaking a semiconductor structurethrough which the conductive path passes in the event the device issubjected to the acceleration. This step may be performed by moving afirst semiconductor body relative to a second semiconductor body inresponse to inertial forces resulting from the acceleration, thesemiconductor structure being coupled at a first end thereof to thefirst body and at a second end to the second body, the movement of thefirst body causing a flexion of the structure, resulting in the breakingthereof.

The device may be a cell phone, and the preselected level may beselected to correspond to an acceleration caused by a drop of the deviceto an unyielding surface from a preselected height. The preselectedlevel may also be selected to be equal to or less than an accelerationsufficient to damage the device.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

For a better understanding of the invention, some embodiments thereofare now described, purely by way of non-limiting examples and withreference to the attached drawings, in which:

FIGS. 1 and 2 are cross-sectional views through a semiconductor wafer insuccessive steps of fabrication in a first embodiment of the processaccording to the present invention;

FIG. 3 is a top plan view of the wafer of FIG. 2;

FIG. 4 illustrates an enlarged detail of FIG. 3;

FIG. 5 is a cross-sectional view of the wafer of FIG. 3 in a subsequentfabrication step;

FIG. 6 is a top plan view of the wafer of FIG. 5;

FIGS. 7 and 8 are cross-sectional views of the wafer of FIG. 6 in asubsequent fabrication step, taken along the planes of trace VII-VII andVIII-VIII, respectively, of FIG. 6;

FIG. 9 is a top plan view of the wafer of FIG. 7, in a subsequentfabrication step, in which an inertial sensor is obtained;

FIGS. 10 and 11 are cross-sectional views of the wafer of FIG. 9, takenalong the planes of trace X-X and XI-XI, respectively, of FIG. 9;

FIGS. 12 and 13 are cross-sectional views through a composite wafer anda die, respectively, obtained starting from the wafer of FIG. 9;

FIG. 14 is a schematic view of the top three quarters of a deviceincorporating the die of FIG. 13;

FIG. 15 is a schematic illustration of an inertial sensor of the typeillustrated in FIGS. 9-13 in an operative configuration;

FIG. 16 is a detail of an inertial sensor obtained according to avariant of the first embodiment of the present process;

FIG. 17 is a top plan view of an inertial sensor obtained according to afurther variant of the first embodiment of the present process;

FIG. 18 is a cross-sectional view of the sensor of FIG. 17;

FIG. 19 is a top plan view of an inertial sensor obtained according to asecond embodiment of the present invention;

FIG. 20 illustrates an enlarged detail of FIG. 19;

FIG. 21 is a top plan view of an inertial sensor obtained according to athird embodiment of the present invention;

FIG. 22 illustrates an enlarged detail of FIG. 21;

FIG. 23 is a schematic illustration of two inertial sensors of the typeillustrated in FIG. 21 in an operative configuration;

FIG. 24 is a cross-sectional view through a semiconductor wafer in aninitial fabrication step of a process according to a fourth embodimentof the present invention;

FIG. 25 is a top plan view of the wafer of FIG. 24;

FIG. 26 illustrates the wafer of FIG. 24 in a subsequent fabricationstep;

FIG. 27 is a top plan view of the wafer of FIG. 26 in a subsequentfabrication step, in which an inertial sensor is obtained;

FIG. 28 is a cross-sectional view through the wafer of FIG. 27, takenaccording to the plane of trace XXVI-XXVI of FIG. 27;

FIG. 29 is a plan view of a detail of an inertial sensor obtainedaccording to a fifth embodiment of the present invention

FIG. 30 is a side view of the detail of FIG. 29; and

FIG. 31 is a side view of the detail of FIG. 29, obtained according to avariant of the fifth embodiment of the present invention.

DESCRIPTION OF THE INVENTION

With reference to FIGS. 1-13, a wafer 1 of semiconductor material, forexample monocrystalline silicon, comprises a substrate 2, on which athin pad oxide layer 3, for example 2.5 μm thick, is thermally grown. Aconductive layer 5 of polysilicon, having for example a thickness ofbetween 400 and 800 nm and a dopant concentration of 10¹⁹ atoms/cm³, isthen deposited on the pad oxide layer 3 and is defined by means of aphotolithographic process. Two T-shaped samples 6 are thus obtained,having respective feet 6 a, aligned with respect to one another andextending towards one another, and respective arms 6 b parallel to oneanother (FIGS. 2-4). The feet 6 a and the arms 6 b of each sample 6 areset in directions identified by a first axis X and, respectively, by asecond axis Y, which are mutually orthogonal (a third axis Z, orthogonalto the first axis X and the second axis Y, is illustrated in FIG. 2). Inaddition, at respective ends of the arms 6 b of both the samples 6anchoring pads 8 are made, of a substantially rectangular shape andhaving a width greater than the arms 6 b. As illustrated in FIG. 4, eachof the samples 6 has a first weakened region 9 and a second weakenedregion 10. In particular, in both of the samples 6, the first weakenedregion 9 and the second weakened region 10 are made as narrowed portionsof the foot 6 a and, respectively, of one of the arms 6 b. In addition,the weakened regions 9, 10 are defined by notches 11 with a circular orpolygonal profile, made in an area of joining 6 c between the foot 6 aand the arms 6 b and traversing the sample 6 in a direction parallel tothe third axis Z. The thickness of the conductive layer 5 ofpolysilicon, the dimensions of the feet 6 a and of the arms 6 b of thesamples 6, and the conformation of the weakened regions 9, 10 determinethe mechanical resistance to failure of the samples 6 themselves. Inparticular, acting on the shape and on the dimensions of the notches 11defining the first weakened region 9 and the second weakened region 10,it is possible to obtain pre-set failure thresholds of the samples 6along the first, second and third axes X, Y and Z. Preferably, all themechanical failure thresholds are basically the same.

Next, a sacrificial layer 12 of silicon dioxide is deposited so as tocoat the pad oxide layer 3 and the samples 6. In practice, the pad oxidelayer 3 and the sacrificial layer 12 form a single sacrificial region inwhich the samples 6 are embedded. The sacrificial layer 12 is thendefined by means of a photolithographic process comprising two maskingsteps. During a first step, first openings 14 are made in thesacrificial layer 12, exposing respective ends of the feet 6 a of thesamples 6, as illustrated in FIG. 5. In a second step of thephotolithographic process (FIG. 6), both the sacrificial layer 12 andthe pad oxide layer 3 are selectively etched, so as to make secondopenings 15, exposing portions of the substrate 2.

Subsequently, a conductive epitaxial layer 16 is grown on the wafer 1,the said layer having a thickness, for example, of 15 μm and a dopantconcentration of 10¹⁸ atoms/cm³. In detail, the epitaxial layer 16 coatsthe sacrificial layer 12 entirely and extends in depth through the firstand the second openings 14, 15 until the samples 6 and the substrate 2,respectively, are reached (FIGS. 7 and 8).

The epitaxial layer 16 is then selectively etched, preferably byreactive-ion etching (RIE), and the sacrificial layer 12 and the padoxide layer 3 are removed. In greater detail, during the step of etchingof the epitaxial layer 16, the following are formed: a mobile mass 18;anchorages 19, provided on the portions of the substrate 2 previouslyexposed by the second openings 15; a plurality of springs 20, connectingthe mobile mass 18 to the anchorages 19; and a ring-shaped supportingstructure 21, which surrounds the mobile mass 18, the samples 6, thesprings 20, and the corresponding anchorages 19 (see FIG. 9, in whichthe sacrificial layer 12 and the pad oxide layer 3 have already beenremoved).

The mobile mass 18 is connected to the substrate 2 by the springs 20,which are in turn constrained to the anchorages 19 (FIG. 11). Thesprings 20, which are per se known, are shaped so as to enableoscillations of the mobile mass 18 with respect to the substrate 2 alongeach of the three axes X, Y, Z, at the same time, however, preventingrotations. The mobile mass 18 is moreover constrained to the substrate 2through the samples 6. In greater detail, the mobile mass 18 has, in amedian portion, a pair of anchoring blocks 22, projecting outwards inopposite directions along the second axis Y. The anchoring blocks 22 areconnected to the end of the foot 6 a of a respective one of the samples6, as illustrated in FIG. 10. In turn, the samples 6 are anchored to thesubstrate 2 through the anchoring pads 8. By controlling the duration ofetching of the sacrificial layer 12 and of the pad oxide layer 3, thesilicon dioxide is in fact removed only partially underneath theanchoring pads 8, which are wider than the feet 6 a and the arms 6 b ofthe samples 6; thus, residual portions 3′ of the pad oxide layer 3,which are not etched, fix the anchoring pads 8 to the substrate 2,serving as bonding elements.

The sacrificial layer 12 and the remaining portions of the pad oxidelayer 3 are, instead, completely removed and, hence, the mobile mass 18and the samples 6 are freed. In practice, the mobile mass 18 issuspended at a distance on the substrate 2 and can oscillate about aresting position, in accordance with the degrees of freedom allowed bythe springs 20 (in particular, it can translate along the axes X, Y andZ). Also the samples 6 are elastic elements, which connect the mobilemass 18 to the substrate 2 in a way similar to the springs 20. Inparticular, the samples are shaped so as to be subjected to a stresswhen the mobile mass 18 is outside a relative resting position withrespect to the substrate 2. The samples 6 are, however, very thin andhave preferential failure points in areas corresponding to the weakenedregions 9, 10. For this reason, their mechanical resistance to failureis much lower than that of the springs 20, and they undergo failure in acontrolled way when they are subjected to a stress of pre-set intensity.

In practice, at this stage of the process, the mobile mass 18, thesubstrate 2, the springs 20 with the anchorages 19, and the samples 6form an inertial sensor 24, the operation of which will be described indetail hereinafter.

An encapsulation structure 25 for the inertial sensor 24 is then appliedon top of the wafer 1, forming a composite wafer 26 (FIG. 12). Inparticular, the encapsulation structure 25 is an additionalsemiconductor wafer, in which a recess 27 has previously been opened, ina region that is to be laid on top of the mobile mass 18. Theencapsulation structure 25 is coupled to the ring-shaped supportingstructure 21 by the interposition of a layer of soldering 29. Next, thecompound wafer 26 is cut into a plurality of dice 30, each diecomprising an inertial sensor 24 and a respective protective cap 31,formed by the fractioning of the encapsulation structure 25 (FIG. 13).

The die 30 is finally mounted on a device 32, for example a cell phone.Preferably, the device 32 is provided with a casing 33, inside which thedie 30 is fixed, as illustrated in FIG. 14. In addition (FIG. 15), theinertial sensor 24 is connected to terminals of a testing circuit 35,which measures the value of electrical resistance between saidterminals. In greater detail, the anchoring pads 8 of the arms 6 b, inwhich the second weakened regions 10 are formed, are connected each to arespective terminal of the testing circuit 35.

In normal conditions, i.e., when the inertial sensor 24 is intact, thesamples 6 and the mobile mass 18 form a conductive path that enablespassage of current between any given pair of anchoring pads 8. Inpractice, the testing circuit 35 detects low values of electricalresistance between the anchoring pads 8. During normal use, the device32 undergoes modest stresses, which cause slight oscillations of themobile mass 18 about the resting position, without jeopardizing theintegrity of the inertial sensor 24.

When the device 32 suffers a shock, the mobile mass 18 of the inertialsensor 24 undergoes a sharp acceleration and subjects the samples 6 andthe springs 20 to a force. According to the intensity of the stresstransmitted to the inertial sensor 24, said force can exceed one of thethresholds of mechanical failure of the samples 6, which consequentlybreak. In particular, failure occurs at one of the weakened regions 9,10, which have minimum strength. In either case, the conductive pathbetween the two anchoring pads 8 connected to the testing circuit 35 isinterrupted, and hence the testing circuit detects a high value ofelectrical resistance between its own terminals, thus enablingrecognition of the occurrence of events that are liable to damage thedevice 32.

According to a variant of the embodiment described, shown in FIG. 16,T-shaped samples 37 are provided, which present a single weakened region38. In particular, the weakened region 38 is a narrowed portion definedby a pair of notches 39, which are oblique with respect to a foot 37 aand arms 37 b of the samples 37.

According to a further variant, illustrated in FIGS. 17 and 18, the twoT-shaped samples 6 are located in a gap 36 between the substrate 2 andthe mobile mass 18 and have the end of the respective feet 6 a in mutualcontact. In addition, both of the samples 6 are fixed to a singleanchoring block 22′ set centrally with respect to the mobile mass 18itself.

The process according to the invention has the following advantages. Inthe first place, for fabrication of the inertial sensor 24, processingsteps that are standard in the microelectronics industry are employed.In particular, the following steps are carried out: steps of depositionof both insulating and conductive layers of material; photolithographicprocesses; a step of epitaxial growth; and standard steps of etching ofthe epitaxial silicon and of the insulating layers. Advantageously, asingle step of thermal oxidation is carried out, and consequently thewafer 1 is subjected to modest stresses during the fabrication process.The yield of the process is therefore high. In addition, the inertialsensor 24 is obtained starting from a standard, low-cost substrate.

The process described consequently enables inertial sensors with failurethreshold to be produced at a very low cost. Such sensors areparticularly suitable for use where it is necessary to record theoccurrence of stresses that are harmful for a device in which they areincorporated and in which it is superfluous to provide precisemeasurements of accelerations. For example, they can be advantageouslyused for verifying the validity of the warranty in the case of widelyused electronic devices, such as, for example, cell phones.

In addition, the inertial sensors provided with the present method havecontained overall dimensions. In inertial sensors, in fact, largedimensions are generally due to the mobile mass, which must ensure thenecessary precision and sensitivity. In this case, instead, it issufficient that, in the event of a predetermined acceleration, themobile mass will cause breaking of the weakened regions of the samples,which have low strength. It is consequently evident that also the mobilemass can have contained overall dimensions.

The use of a single anchoring point between the samples and the mobilemass, as illustrated in the second variant of FIGS. 17 and 18, has afurther advantage as compared to the ones already pointed out, becauseit enables more effective relaxation of the stresses due to expansion ofthe materials. In particular, it may happen that the polysilicon partswhich are even only partially embedded in the silicon dioxide (samplesand portions of the epitaxial layer) will be subjected to a compressiveforce, since both the polysilicon, and the oxide tend to a expand inopposite directions during the fabrication process. When the oxide isremoved, the action of compression on the polysilicon is eliminated, andthe polysilicon can thus expand. Clearly, the largest expansion, inabsolute terms, is that of the mobile mass, since it has the largestsize. The use of a single anchoring point, instead of two anchorages setat a distance apart enables more effective relaxation of the stressesdue to said expansion, since the mobile mass can expand freely, withoutmodifying the load state of the samples.

The inertial sensors obtained using the process described are moreadvantageous because they respond in a substantially isotropic way tothe mechanical stress. In practice, therefore, just one inertial sensoris sufficient to detect forces acting in any direction.

A second embodiment of the invention is illustrated in FIGS. 19 and 20,where parts that are the same as the ones already illustrated aredesignated by the same reference numbers. According to said embodiment,an inertial sensor 40 is made, having L-shaped samples 41. As in theprevious case, the samples 41 are obtained by shaping a conductivepolysilicon layer deposited on top of a pad oxide layer (not illustratedherein), which has in turn been grown on the substrate 42 of asemiconductor wafer 43. Using processing steps similar to the onesalready described, the mobile mass 18, the anchorages 19 and the springs20 are subsequently obtained.

In detail, the samples 41 have first ends connected to respectiveanchoring blocks 22 of the mobile mass 18, and second ends terminatingwith respective anchoring pads 41 fixed to the substrate 2, as explainedpreviously. In addition, notches 42 made at respective vertices 43 ofthe samples 41 define weakened regions 44 of the samples 40.

FIGS. 21 and 22 illustrate a third embodiment of the invention,according to which an inertial sensor 50 is obtained, made on asubstrate 54 and provided with substantially rectilinear samples 51 thatextend parallel to the first axis X. In this case, during the RIEetching step, in addition to the mobile mass 18, two anchorages 52 andtwo springs 53 of a known type are provided, which connect the mobilemass 18 to the anchorages 52 and are shaped so as to preventsubstantially the rotation of the mobile mass 18 itself about the firstaxis X.

The samples 51 have first ends soldered to respective anchoring blocks22 of the mobile mass 18 and second ends terminating with anchoring pads55, made as described previously. In addition, pairs of transverseopposed notches 57 define respective weakened regions 58 along thesamples 51 (FIG. 22).

Alternatively, the weakened regions may be absent.

The inertial sensor 50 responds preferentially to stresses orientedaccording to a plane orthogonal to the samples 51, i.e., the planedefined by the second axis Y and by the third axis Z. In this case, todetect stresses in a substantially isotropic way, it is possible to usetwo sensors 50 connected in series between the terminals of a testingcircuit 59 and rotated through 900 with respect to one another, asillustrated in FIG. 23.

With reference to FIGS. 24-28, according to a fourth embodiment of theinvention, a pad oxide layer 62 is grown on a semiconductor wafer 60having a substrate 61. Next, a conductive layer 63 of polycrystallinesilicon (here indicated by a dashed line) is deposited on the pad oxidelayer 61 and is defined to form a sample 64, which is substantiallyrectilinear and extends parallel to the first axis X (FIG. 25). Thesample 64 has an anchoring pad 65 at one of its ends and has a weakenedregion 66 defined by a pair of notches 67 in a central position.

A sacrificial layer 69 of silicon dioxide is deposited so as to coat theentire wafer 60 and is then selectively removed to form an opening 68 atone end of the sample 64 opposite to the anchoring pad 65.

An epitaxial layer 70 is then grown (FIG. 26), which is etched so as toform a mobile mass 71, anchorages 72, springs 73, and a supporting ring(not illustrated for reasons of convenience). The epitaxial layerextends into the opening 68 to form a connection region 88 between thesample 64 and the mobile mass 71. The sacrificial layer 69 and the padoxide layer 62 are removed, except for a residual portion 62′ of the padoxide layer 62 underlying the anchoring pad 65 (FIGS. 27 and 28). Themobile mass 71 and the sample 64 are thus freed. More precisely, themobile mass 71, which has, at its center, a through opening 74 on top ofthe sample 64, is constrained to the substrate 61 through the anchorages72 and the springs 73, which are shaped so as to prevent any translationalong or rotation about the first axis X. In addition, the sample 65 hasopposite ends, one connected to the substrate 2 through the anchoringpad 65, and the other to the mobile mass 71 at connection region 88, andis placed in a gap 76 comprised between the mobile mass 71 and thesubstrate 61.

In this way, an inertial sensor 80 is obtained, which is thenencapsulated through steps similar to the ones described with referenceto FIGS. 12 and 13.

Also in this case, the use of a single anchoring point between thesample and the mobile mass advantageously enables effective relaxationof the stresses due to expansion of the mobile mass.

According to one variant (not illustrated), the sample is T-shaped, likethe ones illustrated in FIG. 9.

FIG. 29 illustrates a detail of a sample 81, for example a rectilinearone, of an inertial sensor obtained using a fifth embodiment of theprocess according to the invention. In particular, the sample 81 has aweakened region defined by a transverse groove 82 extending in a surfaceof the sample 81 between opposite sides 83 of the sample 81.

The groove 82 is obtained by means of masked etching of controlledduration of the sample 81 (FIG. 30).

Alternatively (FIG. 31), a first layer 85 of polysilicon is depositedand defined. Then, a stop layer 86 of silicon dioxide and a second layer87 of polysilicon are formed. Finally, a groove 82′ is dug by etchingthe second layer 87 of polysilicon as far as the stop layer 86.

Finally, it is evident that modifications and variations may be made tothe process described herein, without thereby departing from the scopeof the present invention. In particular, the weakened regions can bedefined by using side notches in the samples together with groovesextending between the side notches. In addition, the weakened regionscould be defined by through openings that traverse the samples, insteadof by side notches.

All of the above U.S. patents, U.S. patent application publications,U.S. patent applications, foreign patents, foreign patent applicationsand non-patent publications referred to in this specification and/orlisted in the Application Data Sheet, are incorporated herein byreference, in their entirety.

From the foregoing it will be appreciated that, although specificembodiments of the invention have been described herein for purposes ofillustration, various modifications may be made without deviating fromthe spirit and scope of the invention. Accordingly, the invention is notlimited except as by the appended claims.

1. A process for the fabrication of an inertial sensor with failurethreshold, comprising the steps of: forming, on top of a substrate of asemiconductor wafer, at least one sample element embedded in asacrificial region; forming, on top of said sacrificial region, a bodyconnected to said sample element; and etching said sacrificial region,so as to free said body and said sample element, said sample elementbeing physically breakable, following etching of said sacrificialregion, when subjected to a preselected strain.
 2. The process accordingto claim 1, in which the step of forming said sample element comprises:forming a first layer of a first material, which coats said substrate;forming a second layer of a second material, which coats said firstlayer; shaping said second layer, so as to define said sample element;and forming a third layer of said first material coating said firstlayer and said sample element.
 3. The process according to claim 2, inwhich said first material is a dielectric material and said secondmaterial is a conductive material.
 4. The process according to claim 3,in which said first material is silicon dioxide and said second materialis polysilicon.
 5. The process according to claim 1 wherein the step offorming at least one sample element comprises the step of making atleast one weakened region of said sample element.
 6. The processaccording to claim 5, in which the step of making at least one weakenedregion comprises the step of defining a narrowing of said sampleelement.
 7. The process according to claim 6 in which said step ofdefining a narrowing portion comprises forming notches in edges of saidsample element.
 8. The process according to claim 5 in which the step ofmaking at least one weakened region comprises making a groove in asurface of said sample element and extending between opposite edges ofsaid sample element.
 9. The process according to claim 8, in which thestep of making a groove comprises performing an etch of controlledduration of said sample element.
 10. The process according to claim 8 inwhich the step of making a groove comprises: forming a stop layer insidesaid sample element; and etching said sample element until said stopelement is reached.
 11. The process according to claim 1 wherein thestep of forming at least one sample element comprises defining at leastone anchoring pad of said sample element.
 12. The process according toclaim 11, in which the step of etching said sacrificial region isinterrupted before removing residual portions of said sacrificial regionunderlying said anchoring pad.
 13. The process according to claim 1,further comprising making, before performing the step of forming saidbody, at least one first opening through said sacrificial region, whichexposes one end of said sample element, and making second openings,which expose respective portions of said substrate.
 14. The processaccording to claim 13, in which the step of forming said body comprises:growing an epitaxial layer, which extends on top of said sacrificialregion and through said first opening and said second openings; andetching said epitaxial layer until said sacrificial region is reached.15. The process according to claim 14, in which, during the step ofetching said epitaxial layer there are defined anchorages connected tosaid substrate and elastic elements connecting said body to saidanchorages.
 16. A method for manufacturing an inertial sensor,comprising: forming, on a semiconductor substrate, a sample elementhaving a first end coupled to the substrate, and forming the sampleelement being configured to physically break under a preselected strain;and forming, above the semiconductor substrate, a semiconductor materialbody coupled to a second end of the sample element.
 17. The method ofclaim 16 wherein the forming the sample element comprises forming thesample element in a T shape, the first end forming a cross-bar portionof the T and being coupled to the substrate at extreme ends of thecrossbar, the second end extending from a central portion of thecrossbar to form the T.
 18. The method of claim 16, further comprisingforming an additional sample element having a first end coupled to thesubstrate, a second end coupled to the semiconductor material body, andconfigured to break under the preselected strain.
 19. The method ofclaim 16, further comprising forming a weakened region on the sampleelement, and wherein the sample element is configured to break at theweakened region under the preselected strain.
 20. The method of claim 19wherein the weakened region comprises a narrowed region of the sampleelement.
 21. The process of claim 1, comprising forming, in saidsacrificial region, a via over said sample element, and wherein saidstep of forming the body comprises connecting the body to said sampleelement through said via.
 22. The process of claim 1 wherein said stepof forming said sample element comprises forming a plurality of sampleelements, of which said sample element is one, each of the plurality ofsample elements being coupled at a respective first end to thesemiconductor substrate and at a respective second end to saidsemiconductor material body.
 23. The process of claim 1, furthercomprising forming, in said semiconductor wafer, a test circuitconfigured to detect a break in said sample element.
 24. The method ofclaim 16, further comprising forming a circuit on the semiconductorsubstrate, configured to subject the sample element to a voltage such asto cause an electric current to flow through the sample element.
 25. Themethod of claim 24 wherein the circuit is further configured to detectthe current flowing in the sample element.
 26. A process for thefabrication of an inertial sensor with failure threshold, comprising:forming, on top of a substrate of a semiconductor wafer, a conductivesample element coupled at a first end thereof to the substrate andshaped so as to be subject to a stress when a second end thereof isoutside a relative resting position with respect to the substrate, andwherein configuring the conductive sample element is configured so as toundergo a physical failure in a controlled way when subjected to astress of pre-selected intensity; and forming a body connected to thesecond end of the sample element.
 27. The process of claim 26, furthercomprising forming, in the semiconductor wafer, a test circuitconfigured to detect a change in electrical resistance of the sampleelement.
 28. The process of claim 26 wherein the step of forming aconductive sample element comprises forming a plurality of sampleelements of which the sample element is one, each of the sample elementbeing coupled at respective first ends to the substrate and atrespective second ends to the body, and each of the sample elementsbeing configured to undergo failure in a controlled way when subjectedto the stress of pre-selected intensity.
 29. The process of claim 26wherein the step of forming a sample element comprises the step ofmaking a weakened region of said sample element.
 30. The method of claim16, comprising forming the semiconductor material body with a masssufficient that, if the inertial sensor is subjected to a predeterminedacceleration, the mobile mass will subject the sample element to thepreselected strain, causing the sample element to break.
 31. The methodof claim 16 wherein the forming the sample element comprises forming thesample element in an L shape, and forming the sample element to break ata vertex of the L when the sample element is subjected to thepreselected strain.
 32. The method of claim 16 wherein the forming thesample element comprises forming the sample element in a rectilinearshape.
 33. The method of claim 16, comprising forming a plurality ofspring elements, each coupled at a respective first end to thesemiconductor substrate and at a respective second end to thesemiconductor material body, and forming each of the spring elements tohave a mechanical resistance to failure that is greater than that of thesample element.